The use of multiple antennas at the transmitter promises to deliver important performance improvements in terms of saving power, improving link quality, and increasing capacity of wireless systems in general. For these reasons Downlink Transmit Diversity mode has been included in the initial version of the WCMDA 3GPP standard in Release 99 [3GPP TS 25.211, “Physical channels and mapping of transport channels onto physical channels (FDD)”, September 2002, Section 5.3.1]; and in Release 7 the MIMO (Multiple-Input-Multiple-Output) mode, which uses multiple antennas at both the User Equipment (UE) and at the Base Station (BS), has been adopted [3GPP TS 25.214, “Physical layer procedures (FDD)”, December 2008, Section 9]. More recently the Long Term Evolution (LTE) or Release 8 of 3GPP standard has also adopted multiple transmit antennas techniques to boost performance [3GPP TS 36.211, “Physical channels and modulation”, March 2009].
Two main categories of multiple transmit antenna techniques can be distinguished: (i) multiple transmit antenna techniques without use of channel state information (CSI), and (ii) multiple transmit antenna techniques with the use of CSI.    (i) Multiple transmit antennas techniques that do not use the CSI are non-adaptive (see FIG. 1). Among this family of techniques are the Space-Time Codes and particularly the well know Alamouti scheme used for the two transmit antenna case where the space-time coding aims at capturing the diversity of the antennas in order to reduce the likelihood of the channel being in deep fade.    (ii) Multiple transmit antenna techniques that do use the CSI are adaptive (see FIG. 2). The adaptation comprises adjusting the relative phase and/or amplitude of a signal as transmitted from the different transmit antennas. A good example is the popular beamforming whereby the multi-antenna transmission is steered in space in a way to achieve maximum received power.
Naturally, adaptive techniques which use CSI achieve better performance gains than those that are agnostic to the CSI; but on the other hand non-adaptive techniques show more robustness especially in environments where the CSI is partial or inaccurate, for example in the case of high speed channels.
Recently hybrid approaches like long-term precoding have been proposed. Those techniques try to combine non-adaptive coding methods with the use of partial CSI like the long-term correlation of transmit antennas to achieve a compromise between the performance of adaptive techniques and the robustness of the non-adaptive ones.
Current adaptive techniques require the availability of CSI. Some methods have been proposed that exploit the channel reciprocity (between the transmit link and receive link) to provide CSI to the transmitter, but those techniques are only applicable when Time Division Duplex (TDD) is used and even in this case a challenging calibration is required between the transmit and receive links. It remains that in most of the existing Frequency Division Duplex (FDD) standards availability of the CSI at the transmitter is only achieved by feedback from the receiver through the reverse link.
The feedback of the CSI from the receiver to the transmitter results in a significant overhead on the reverse link, and this is especially true for cellular systems where the number of users can be significant and therefore the feedback has to be performed for all the users, which greatly reduces the system capacity. Additionally, the introduction of the feedback by itself leads to an increased complexity of the system.
By way of example, consider the problem of two transmit (Tx) antennas and one receive (Rx) antenna, in which the phase at the second transmit antenna is adapted to achieve the highest receive power.
The commonly held point view is that, for the adaptation to work, then CSI in the form of the optimal phase between the two antennas has to be fed back from the receiver to the transmitter on the reverse link (see FIG. 3). This not only means that a specific feedback mechanism needs to be designed and included in the reverse link, but also that the receiver must have the capability to estimate the channel coming from each antenna (which requires a different pilot sequence per antenna). This makes the introduction of such a scheme complicated, and furthermore increases the signalling overhead.